This invention relates to an on-board sensing element and method for using the same to measure the volatility of a sample of non-ethanol gasoline by measuring the change in capacitance of the sensing element as a function of time and temperature and using the measurements to estimate the driveability index (DI) of the sample.
It is known in the art relating to automotive engines, that the key gasoline characteristic of good driveability is volatility. Volatility is especially important at the time an engine is started because liquid gasoline must evaporate and mix with air to form a combustible mixture. If too little gasoline is added, the engine will not start. If gasoline beyond that needed to initiate combustion is added, then extra hydrocarbons from an unburned portion of the gasoline are found in the exhaust. Moreover, because gasoline sold in the United States varies in volatility, there is a tradeoff in engine design between low hydrocarbon emissions and good driveability with low volatility fuel.
To describe the effect of gasoline volatility on the cold start and warmup driveability of a vehicle, a driveability index (DI) has been developed. For gasoline that does not contain oxygenates such as ethanol or methyl tertiary-butyl ether (MTBE), the definition of DI is based on a laboratory test (ASTM D86) in which a sample of gasoline is distilled as its temperature is raised. The fraction distilled is measured as a function of temperature and the equation:
DI=1.5T10+3T50+T90
where Tx is the temperature in degrees Fahrenheit at which x % of the gasoline sample has been distilled.
One known way to estimate DI is by measuring the fuels infra-red transmission spectrum. While this approach has proven useful in refineries where the feedstocks are known, it has not been accepted as an accurate way to characterize the DI of finished gasoline in the field.
It is particularly desirable to estimate DI on-board a vehicle. To provide customer satisfaction, engines are calibrated to reliably start with fuel of the lowest volatility. This is done by increasing the amount of fuel in the air/fuel mixture. Consequently, for most starts, the engines air/fuel ratio is richer than optimum. Some of this extra gasoline passes unburned into the exhaust. This is particularly detrimental at the time of a cold start because the catalytic converter is too cold to be active. The added hydrocarbon concentration is typically emitted to the environment.
Estimating DI on-board would permit the air/fuel ratio to be more precisely controlled. The engine would be calibrated to reliably start while extra fuel would only be added when needed to compensate for fuel volatility. On the average, less fuel would be used for cold starts resulting in a decrease in fleet average exhaust hydrocarbon emissions. This decrease in air pollution is an important environmental benefit.
The present invention provides a method for using an on-board sensor having a sensing element for calculating the fuel DI number from measured changes in electrical capacitance, which is representative of the volume of the fuel filled sensing element as the sensing element is heated to evaporate the fuel within it. Both the heated sensing unit and the standardized test (ASTM D86) measure fuel distillation or vaporization. However, due to different thermal mass and structure of the two systems, measured distillation curves are quite different. Since the ASTM D86 test is the industrial standard, it is necessary to calibrate the measured results from the heated sensing element to match the results obtained from the industrial standard.
Fuel samples are provided to measure the driveability index (DI). Each fuel sample is divided into two containers. One container is used for the ASTM D86 measurements and the other container is used to fill the sensing element for the sensor measurements. The required temperature information is obtained from the D86 measurements to calculate the DI number.
The sensing element is then heated in a controlled environment so that the sensor""s change in capacitance and temperature over time is measured. Using mathematical analysis, the relationship of the sensing element data with the standard D86 test data is calibrated to provide correlation measurements. The correlation measurements are stored to the engine controller of the vehicle, which calculates DI as needed. The calculated value of DI is stored for the next cold start where it may be used for setting the desired air/fuel ratio at the time of starting.
Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.